Tuesday, February 16, 2010

Pigeon Week Part 3: Peregrine Attacks, Plumage and Feral Future

When you think “pigeon”, you probably have an image in your head that looks something like this: dark head and neck, light gray breast and belly, black-tipped tail, and 2 black stripes on each wing. But if you slow down a bit and check out a group of pigeons around town, you’ll notice that many look quite different.

The “standard” type I described above is a called a Blue-bar, even though the bars on its wings aren’t blue, and it’s closest in coloration to wild rock pigeons. Another very common color pattern is called the Blue Checker. At first glance they appear similar to Blue-bars, but their wing bars are generally a bit wider, and the feathers between bars and neck are spotted with black “checks.” You’ll also notice that some pigeons are very dark, almost a black slate color except for more metallic-y looking feathers on the neck, and this pattern is called Spread.

Every once in a while you’ll see a white pigeon. Though a few white pigeons are true albinos, most are not; they’re just colored white, and their non-albinism is almost always belied by a few spots of darker color. Another noticeable pattern is pigeons that have spots of white in key locations only, such as the head, or wing-tips, and this pattern is called Pied. Yet other pigeons have red or reddish-brown feathers, and depending on the distribution of such feathers, some of these are known as Red or Red-bar.

So there are a number of basic color patterns among pigeons, with many subtle gradations between the basic types, and therefore unlike almost all of the other wild animals we see daily around town- magpies, crows, starlings, mule deer, squirrels, raccoons- individual pigeons are fairly easy to tell apart.

Tangent: In fact, over the 2 weeks that I’ve been observing and photographing my favorite flock at the strip mall on the NW corner of 1300 East and 8600 South, I’ve actually come to recognized several individuals, and noticed that they almost always perch at the exact same location. This White (pic right) for instance, is always just under the West-facing eave atop of the Indian/Chinese restaurant building. The Pied pictured below is on the East side of the roof across the lot, the Northernmost of the 2 pigeons perched there. Seriously, if you live in Salt Lake Valley and you drive over there right now, I’ll bet money that’s exactly where they’ll be.

So while it’s nice that they’re easy to tell apart, it raises an interesting question: why so many colors/color patterns? Wild pigeons, true wild Columba livia, from the Old World, are basically all Blue-bars. The pigeons we see are feral pigeons, meaning that they’re descended from domestic pigeons who at some point escaped captivity. Human breeders have bred pigeons for many characteristics, including color, so it’s easy to see where feral pigeons got their color: from their domesticated ancestors.

But that doesn’t explain why they’ve kept their colors. Most feral animal populations, after successive generations, tend toward a similar coloration pattern. One might argue that not enough time has passed for colors to “average out”, but feral pigeons have been thriving in North America for hundreds of generations, and in the Old World for thousands. The answer isn’t obvious. A number of raptors hunt pigeons, and in the wild animals with unusual coloration tend to be singled out of a flock or pack more often by predators. So why don’t modern feral pigeons look more alike?

There are some trends in pattern and coloration. In Eurasia, the farther North you go between about 40 and 60 degrees in latitude, the darker feral pigeons tend to be. The trend is too high for chance*but it’s not clear what the Northern selection pressure for darker pigeons is.

Another trend is that urban populations seem to show more color variety- and specifically more whites- than rural, “wild”-living feral populations. It’s suspected that the relative dearth of traditional raptor-predators in many urban areas may ease selection pressure toward wild-type coloration, and non blue-bar color-patterns (think Checker or Spread) may even have advantages against other urban-specific threats, such as humans, dogs and cats.

While the possible benefits and liabilities of all color patterns under all conditions aren’t known, the benefit of at least one pattern is. Many feral pigeons- and many truly wild pigeons- have a distinctive white patch on their lower backs, just above the tail. When the pigeon is on the ground, the patch is mostly covered by the folded wings and not easy to see (pic left). But if you catch a glimpse of a pigeon flying away from you, it’s immediately obvious (pic below, left).

Over a 7 year period, researchers observed several flocks of feral pigeons, numbering roughly 5,000 total, in the Davis, California area, roughly 23% of whom had this white patch. During this time they witnessed nearly 1500 attacks on these pigeons by 5 Peregrine Falcons (pic right, not mine) in the area, 3 of whom were still juveniles when the study began. Peregrines are regular and highly effective predators of pigeons, swooping down from above and behind and capturing them in mid-air. The adults in the study made a successful kill in 40% of attempts, while the juveniles were successful 19% of the time.

Of the ~5,000 pigeons, about 23% had the white rump-patch I just described above. So of the pigeons killed by the Peregrines over the 7 year study, guess what percentage had the white rump-patch. Probably around 23%, right? Guess again- it was 2%. That’s right, if you were a pigeon in Davis, California over the last decade, having white feathers on your rump made you 10 times likelier to successfully evade a mid-air Peregrine attack.

Actually, that’s not quite right. I should say, having white feathers on your rump made you 10 times likelier not to get killed by a Peregrine, period. Because while the juvenile Peregrines attacked White-Rumpers about as often as they attacked any pigeon- roughly 20% of the time, adult Peregrines only bothered to even try to catch them about half as often- only 10% of the time. The adults clearly learned that White-Rumpers were harder to catch, and so most of the time focused on other, easier-to-catch color patterns.

What was going on? Could White-Rumpers be faster, smarter or more maneuverable than other pigeons? Apparently not; the researchers captured over 700 White-Rumpers and Blue-bars and switched their rump-feathers, whereupon the (former) White-Rumpers lost their advantage and the Blue-bar-transplants suddenly became 10 times harder to catch!

All About Peregrine Attacks

To understand the advantage of the white rump-patch, we have to know a bit about how Peregrines catch pigeons. Peregrines target pigeons from high above. When they lock into a target, they go into a dive, reaching speeds of over 200 mph*. The falcon levels off from its dive on the level of the targeted pigeon, at a point 20-100 meters behind it. Pigeons are themselves fast- racing pigeons have been clocked at 94 MPH- but not fast enough. The Peregrine closes from behind and catches the pigeon in its talons. Well, 40% of the time, anyway.

*Fastest animal on the planet.

The other 60% of the time, the Peregrine misses, and it misses because the pigeon takes evasive action. The most common evasive maneuver is a roll, which the pigeon effects by dropping one wing and quickly rolling 360 degrees over and down to a plane below the Peregrine’s attack trajectory.

To be able to catch the pigeon, the Peregrine must quickly modify its (very high-speed) trajectory, and to do so it must observe and react to the pigeon’s wing-drop, which marks the beginning of the roll. But for some reason, the white rump-patch appears to make the Peregrine less able to detect the wing-drop and correct course in time. Somehow this bright white spot distracts the falcon’s attention from the wings, and it misses the pigeon.

Wow. White rump-patches sound awesome! Why don’t all feral pigeons have them? Well, not all feral pigeons are regularly exposed to Peregrines, and for a few decades, hardly any were. In non-Peregrine environments, it’s not obvious that they offer any other benefits, and who knows, they may be a liability when confronted with other predators. In the 1960s Peregrines came close to extinction, and disappeared completely from the Eastern US and Canada. DDT weakened their eggshells, which routinely cracked during incubation. But now they’re making a come-back, and re-colonizing much of their former range. And, like pigeons, they seem to adapt well to cities, nest on high buildings and attracted by the availability of prey (pigeons!) It may well be that the return of the Peregrines is creating a selection pressure for white rump-feathers, and in fact over the course of the Davis study the researchers noticed an increase in the occurrence of this color-pattern.

Extra Detail: The birds that had the roughest time with DDT were those which, like Peregrines or Bald Eagles, are at or near the top of the food chain. DDT (C14H9Cl5) is a chlorinated hydrocarbon, and accumulates in the fatty tissues of creatures who ingest it. The concentration increases as other creatures consume those contaminated tissues, resulting in much higher concentrations in top predators. Seabirds also had a tough time, as they ate large numbers of contaminated fish. Brown Pelicans in particular came close to extinction but are now recovering nicely.

That’s just one pattern and one story. In the many environments- both rural and urban- which feral pigeons inhabit, there may be other color-pattern-benefit stories we haven’t yet figured out.

So it’s time to wrap up Pigeon Week, and I want to do so by returning to the issue I keep mentioning but skirting past- their “feral-ness.”

Wild pigeons were first domesticated, for food, some 5,000 years ago, making them the world’s first domesticated bird. They’ve been bred in captivity ever since, for food, as well as communication and sport (racing), and for as long as people have been keeping pigeons, pigeons have been escaping from them. We know that many ancient cities had flocks of feral pigeons, including Rome, where they roosted atop some of the major buildings and monuments.

Pigeons were introduced to North America in 1606 by French colonists in Nova Scotia, and starting escaping shortly thereafter. Today in pretty much any major city in the world, you can easily find feral pigeons. They’re everywhere, and it’s hard to think of a bird that’s doing better in the human-altered world of today.

Which is ironic, because wild pigeons may be close to extinction. Not last-guy-dropping-dead extinction, but genotypic extinction. As feral pigeons expand their range and multiply, they’re both cross-breeding with, and simply out-breeding, wild pigeons when coming into contact with them. By the end of this century, there could well be no truly wild pigeons anywhere in the world.

This isn’t as weird or unusual as it may sound. An example we see all the time is ducks. The common duck you see in parks or on ponds is the Mallard, Anas platyrhynchos. Mallards are another exotic, introduced to North America by Europeans, and the wild ducks we see here today are- yes, that’s right- feral. Mallards interbreed with many native ducks when encountered, and it’s believed at least some native North American ducks, such as the American Black Duck, A. rubripes, may already be genotypically extinct, surviving today only in hybrid form. Similar possible gentotypic extinctions are happening or have happened with native ducks in Hawaii, South Africa, and elsewhere. Genotypic extinction isn’t just a bird thing either; as we saw almost 2 years ago, exotic Old World triploid Dandelions present a similar threat to the remaining North American species.

Tangent: There’s a weird parallel with some human populations (though I want to be careful and make clear that the differences between disjunct populations of modern humans are nowhere near the level to be considered separate “species.”) Truganini, the last full-blooded Tasmanian aboriginal, died in 1876, a tidbit referenced in a number of sources as the “extinction” of the native Tasmanians. This “extinction” moniker has ticked off a number of modern-day Tasmanians, who while not full-blooded, claim Tasmanian ancestry. It’s probably more accurate to say that Tasmanian aboriginals are genotypically extinct.

Nested Tangent: While I’m diving into borderline-taboo human ancestry topics, I can’t help but noting another parallel- this one more inspiring. Successful feral species make me think of human populations whose ancestors escaped from bondage. The one we’re probably all most familiar with is the Israelites’ escape from Egypt, but recent (and historically clearer) stories are right in our back yard.

The recent earthquake in Haiti is yet another in a series of tragic mishaps to befall that nation, and nowadays when we think of the place, we regard it mostly with sadness and pity. That’s sad in and of itself, but it’s doubly sad in that it obscures the human triumph of Haiti’s founding: an enslaved people who united, rose up, successfully overthrew their masters, and founded a nation- a story unique in the hemisphere (maybe the world?), something accomplished by no other modern people. And while they didn’t give rise to a nation-state, the stories of the Jamaican Maroons or the Garifuna of Central America are no less inspiring. For me, these are some of the most stirring stories of recent human history and at some level I envy these peoples the pride they must feel of the sacrifices and accomplishments of their ancestors, even if I don’t envy their present circumstances.

But the mallards, while interfertile, are a different species; feral pigeons are the same species as wild pigeons. Why would they outbreed them?

Domesticated animals are bred for traits desirable to their human masters, a process called artificial selection, which has produced the array of domestic pets and livestock we see today. Domestic pigeons have been bred for several traits, one of which is fecundity. Domestic pigeons regularly rear as many as 18-20 squabs per year. I mentioned in part 2 that feral pigeons in temperate latitudes breed year-round. But what I didn’t mention is that wild pigeons don’t; year-round breeding is a legacy of human domestication, and their feral descendants, with their human-induced powers of fertility, are steadily out-breeding their wild cousins.

Tangent: I’m fascinated by feral animals, their origins and lifestyle. Sometime I wonder if, long after we humans disappeared, if evolution were to somehow eventually bring some feral species- pigeons, ducks, dogs, horses to a state of human-like intelligence and self-awareness, and these future pigeonoids or duckanoids or whatever were to investigate and research and come to understand their evolutionary past, including their domesticated origins, how might they regard us? As protectors? Parasites? Angels? Gods?

That’s Pigeon Week. I hope you enjoyed it, and that the next time you notice a flock of pigeons you don’t just dismiss them as “feathered rats.” They see and sense a world beyond the one you or I see, their family lives are exemplary, and their story inspiring. Pigeons are way cool.

This series feels like kind of a milestone for me in this project. We’ve checked out some several of the most common, taken for granted birds around, including the 3 Big Exotics (Pigeons, House Sparrows and Starlings) as well as our own local signature corvid (Magpie.) In each case their stories have turned out to be way, way more interesting than I ever would have guessed when I decided to check them out. And that’s really been the big epiphany of this whole project for me: that everything, every living creature, not just the rare or unique or exotic, but the most common, “boring”, ordinary, everyday creatures, things like Pigeons and Dandelions and Crabgrass and Box Elder Bugsand Yellowjackets have these amazing stories behind them. It feels good to understand and connect these stories. Sort of like, after all these years, the world is starting to make sense.

Note about sources: It was a pleasure to research a topic with so many excellent sources available. I’m extremely grateful to my friend and fellow nature blogger KB for helping me track several of them down. Much of the info for Parts 1 and 2 came from this site and this site, which also provided much of the plumage info for Part 3. This paper provided the data of the Davis, California plumage study referenced in Part 3. This paper detailed the possible genotypic extinction of wild pigeons. Additional info came from Colin Tudge’s outstanding book The Bird, and from the Cornell ornithology website. (And I’m forgetting probably a dozen other very helpful sites.)

12 comments:

Truly Awesome, Watcher... thanks for all the light you've shown on pigeons. You're right, we'll never see them quite the same again! And now we even have names for the color patterns.

Earlier in Pigeon Week, I was struck with nostalgia (we used to raise ring-necked doves at the museum, so it brought back memories; that's when I learned about crop milk). This post is great, and the graphics! Oh my...

But you owe us one more-- you haven't yet answered the most puzzling of all pigeon questions. Someone asked me today, even (and in fact we had been talking about crows when it came up out of the blue). That is: how come we NEVER see baby pigeons? Here at Foothills base camp, we see all kinds of juvenile birds, but not pigeons. What's the story?

Sally- The whole “Where are baby pigeons?” thing is a super-common question; google it and you’ll find all sorts of answers. The likely explanation is the nests are generally pretty inaccessible, and the squabs don’t leave the nest before they can fly. But check out Ray from Australia’s comment- as he points out, there’s likely a month-old “squeaker” in my first shot!

Christopher- Thanks! Made the correction.

Ray- Thanks so much for the info and corrections, especially on the squeaker! I knew little about Pigeons prior to this last week, including- I’m embarrassed to say- that there was even such a thing as pigeon racing...

On another note, apparently there are now not just one, but two Australians who read this blog. Crikey! How cool is that?*

*Hey I ‘m just glad there are 2 people period who read this thing. Not counting my mother. (Yes, she’s still reading. She doesn’t comment, but she emails me corrections on hometown place-names and Greek grammar. Hi Mom.)

Hi Watcher, just wanted to say how wonderful it was to read your posts on pigeons! They don't deserve the bad image some people have of them - something you've pointed out in the lovely things you've written about pigeons. And very well researched too! :) Thank you!Ps. Hope you don't mind me writing about your posts on my blog. Just spreading the word! :)

Watcher, Well done. This thread has been linked to one of the best pigeon forum`s in Australia, so if I`m number three, i suspect there will a few more that have ingested your words of wisdom. Peregrine numbers in Australia, have increased ten fold in the last few years and the pigeon racing fraternity are looking for any advantage they can get, to out wit the most magnificent and proficient predator in the skies. Some of us might have to get the white paint out of our sheds and do some touch ups on our pure blue bloods and once again, enjoy our magnificent sport, without getting ulcers from worry that our little warriors will make it back home and not become dinner.

Indeed, an informative article but with respect, anonymous, there's no evidence that pergerines have increased "ten fold in the last few years" - in fact that is biologically impossible. Attacks on domestic pigeons are not a good measure of changes in numbers because availability of other prey and numbers of pigeons available to be attacked have a huge influence. All the science, and there's alot on pergerines in Australia, is that there has been no measurable change in pergerines for many years. I look forward to seeing more passive methods of reducing losses.